1. Field of the Invention
The present invention relates to a fuel control apparatus for an internal combustion engine wherein a pressure in an intake air pipe (hereinbelow, referred to as an intake air pipe pressure) is estimated and a fuel injection quantity is calculated on the basis of a value determined by the intake air pipe pressure.
2. Discussion of Background
A conventional fuel control apparatus for an internal combustion engine will be described with reference to FIG. 1 which is also used for describing an embodiment of the present invention.
FIG. 1 shows an engine which is subjected to a speed density type SPI (single-point-injection) fuel control, wherein a reference numeral 1 designates an internal combustion engine, a numeral 3 designates an intake air pipe, a numeral 3b designates a throttle body, a numeral 4 designates a throttle valve, a numeral 5 designates a sensor for detecting a degree of opening of the throttle valve (hereinbelow, referred to as a throttle sensor), a numeral 6 designates a bypass air passage, a numeral 7 designates a wax type first idle air valve (hereinbelow, referred to as an FIA valve) used for fast idling, a numeral 11 designates an ON/OFF type air conditioner idle-up solenoid valve (hereinbelow, referred to as an ACIUS valve) used for air conditioning idling-up, a numeral 12 designates a switch for air conditioner, a numeral 13 designates a duty-controlled idle speed control solenoid valve (hereinbelow, referred to as an ISC solenoid valve) used for adjusting a revolution number in idling, a numeral 14 designates a pressure sensor, a numeral 15 designates a fuel injector, a numeral 16 designates an ignition coil, a numeral 18 designates an exhaust pipe, a numeral 20 designates a branch pipe for exhaust gas, a numeral 21 designates a control valve for recirculating exhaust gas (hereinbelow, referred to as an EGR valve), a numeral 22 designates an exhaust gas recirculating port, and a numeral 23 designates a water temperature sensor.
Exhaust gas recirculated into the exhaust gas recirculating port 22 contains a water component. Accordingly, it is necessary to form a pressure introducing port at the upper stream side to the exhaust gas recirculating port 22 in order to prevent the water component from entering into the pressure sensor 14. However, when the pressure introducing port is formed in the main passage of the throttle body 3b, fuel may enter through the pressure introducing port. Therefore, the pressure introducing port for the pressure sensor 14 is formed in the bypass air passage 6 in order to prevent the invasion of the water component and fuel.
A numeral 25 designates a control unit which receives signals from the throttle sensor 5, the pressure sensor 14, the ignition coil 16, the water temperature sensor 23 and so on; processes the signals, and actuates the ISC solenoid valve 13 and the fuel injector 15.
The operation of the conventional fuel control apparatus will be described with reference to an operating flow chart in FIG. 22 which is stored as a form of program in the control unit 22.
At Steps S1, S2, S3 and S4, an engine revolution number, an intake air pipe pressure, a cooling water temperature and a degree of opening of a throttle valve are respectively detected, and digital signals indicating an actual revolution speed Ne, an intake air pressure value Pb', a cooling water temperature value WT and a throttle opening value .theta. are sequentially read at each time of detection.
At a Step S5, controlled variables for the ISC solenoid valve 13 are calculated so as to control the revolution number in the idling operation, if there is found an idling operation on the basis of the operational conditions such as an engine revolution number, a throttle valve opening degree and so on.
At a Step S6, a volumetric efficiency C.sub.EV (Ne, Pb') is calculated by using a two-dimensional map prepared by using data of the revolution number Ne and the intake air pipe pressures Pb'.
At Step S7, an engine warming-up increment coefficient C.sub.WT (WT) is calculated by using the obtained coOling water temperature value (TW).
At Step S8, a driving time .tau. for driving the fuel injector 15 is calculated in accordance with an equation .tau.=K.times.Pb'.times.C.sub.EV .times.C.sub.WT (where K is a constant).
After Step S8, sequential operation is returned to Step S1 to thereby repeat the above-mentioned steps.
In the conventional fuel control apparatus having the above-mentioned construction, a pressure loss is resulted because the bypass air passage 6 is thin. With the result of this, a pressure difference results between an intake air pipe pressure at the inlet port of a conduit for introducing pressure for the pressure sensor 14 and an intake air pipe pressure at the exterior side of the outlet port of the bypass air passage 6, whereby the pressure sensor 14 detects a pressure higher than the actual intake air pipe pressure. In particular, the pressure difference is greater when the specific gravity of air flowing the bypass air passage 6 is large, or a flow rate of air in the passage 6 is large. More particularly, the pressure difference becomes the maximum when the engine 1 is at a low temperature. Therefore, when a fuel injection quantity is calculated on the basis of an intake air pipe pressure value Pb' obtained by the pressure sensor 14, the air-fuel ratio tends to be at a rich side since the fuel injection quantity is calculated to be more than the quantity corresponding to the actual intake air pipe pressure value. In particular, when the engine is at a low temperature, the air fuel ratio becomes excessively rich, which causes the reduction of fuel consumption efficiency, drivability and so on.